IL-27 is a Th1 cytokine that induces IFN-g secretion along with IL-12. However, it differs from IL-12 in that lack of IL-27 signaling has been associated with excessive responses to infectious agents. To explore the mechanism of this phenomenon we determined the effect of IL-27 on T cell apoptosis. We found that stimulation of T cells with a polyclonal activator in the presence of IL-27 and anti-Fas or Fas L led to marked increased apoptosis. This IL-27-enhanced apoptosis was accompanied by increase activation ?induced cell death and increased apoptosis associated with cytokine withdrawal (IL-2 withdrawal). In contrast, TNF- or Trail-induced apoptosis was not significantly affected. IL-27 is not simply mimicking the effects of either IL-2 or IFN-g since it enhances Fas-mediated apoptosis in the presence of excess amounts of these cytokines. In studies of the mechanism of IL-27-induced apoptosis, we found that IL-27 had no significant effect on FasL expression , and by quantitative rt-PCR it has only minor effects on any of a variety of intracellular factors shown previous to be involved in apoptosis such as Bcl-2 , Bcl-xL, FADD, FLIP and caspase 8. In addition , by rtPCR it had only minor effects on survivin, IAP proteins or GADD45 isoforms. Finally, IL-27 enhanced apoptosis was not significantly affected by the presence of a methyl1-beta-cyclodextrane (MBCD) an agent that interferes with the assembly of the death-inducing signaling complex in cell membrane rafts. While these studies do not identify the mechanism of IL-27 enhancement of apoptosis, they do establish the the reality of this phenomenon and therefore one of the reasons IL-27 deficiency leads to over-exuberant immune reponses. As such, defects in IL-27 secretion could account for some types of autoimmunity.